Restore "Edit this Page" link for editable docs pages (#20900)

* Restore "Edit this Page" link for editable docs pages

Bring back the right-rail "Edit this Page" link that was dropped in #19855,
but only surface it on pages a reader can actually edit in this repo.

Rather than the stale path-prefix checks the old version used (the CLI and
reference-doc paths have since moved), suppression is driven by signals that
stay correct as content moves:

- generated pages are detected by the "Auto generated by ..." marker they
  carry (the same signal scripts/lint/lint-markdown.js already trusts),
- registry package docs are excluded since they're built from pulumi/registry,
- no_edit_this_page: true remains a manual per-page opt-out.

The link uses the docs semantic color tokens so it reads correctly in both
light and dark mode.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* Anchor registry edit-link guard to the path prefix

The previous strings.Contains "registry/" check matched any permalink
containing that substring — including /docs/idp/concepts/private-registry/,
which wrongly suppressed the edit link. Use hasPrefix on .File.Path so only
the top-level registry/ section (built from pulumi/registry) is excluded.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* Drop the registry edit-link guard

Everything under /registry is built and deployed from pulumi/registry, so no
page in this repo renders through this partial with a registry/ path. The
guard was dead code; remove it. Suppression now rests on the auto-generated
marker and the no_edit_this_page opt-out.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: cnunciato

layouts/partials/docs/table-of-contents.html

@@ -57,6 +57,22 @@ <h2 class="no-anchor">On this page</h2>
 {{ end }}
 <ul class="p-0 list-none table-of-contents-feedback">
     {{ if .File }}
+        {{/* Hide "Edit this Page" for pages that aren't editable here: generated
+             pages carry the "Auto generated by ..." marker (the same signal the
+             linter uses), and no_edit_this_page is a manual opt-out. */}}
+        {{ $generated := findRE "###### Auto generated by" .RawContent 1 }}
+        {{ if not (or $generated $.Page.Params.no_edit_this_page) }}
+            <li>
+                <a
+                    data-track="edit-page"
+                    class="text-docs-fg-muted hover:text-docs-fg text-xs no-underline"
+                    href="{{ $.Site.Params.contentRepositoryURL }}/edit/{{ $.Site.Params.contentRepositoryBaseBranch }}/content/{{ .File.Path }}"
+                    target="_blank"
+                >
+                    {{ partial "icon.html" (dict "name" "pencil-simple" "class" "mr-2") }}Edit this Page
+                </a>
+            </li>
+        {{ end }}
         <li>
             <a
                 data-track="request-change"